1. Field of the Invention
This invention relates to the structure and fabrication of heterojunction bipolar transistors, and more particularly to the use of selective area ion implantation in the formation of such transistors.
2. Description of the Related Art
Heterojunction bipolar transistors (HBTs) are attracting interest currently because of their potential for high speed and high frequency applications. Perhaps the two most popular current fabrication techniques for high speed HBTs are described in the following articles: the first by Chang et al., "AlGaAs/GaAs Heterojunction Bipolar Transistors Fabricated Using a Self-Aligned Dual-Lift-Off Process", IEEE Electron Device Letters, Vol. EDL-8, No. 7, July 1987, pages 303-5; and the second by Nakajima et al., "High Speed AlGaAs/GaAs HBTs with Proton Implanted Buried Layers", IEDM Tech. Dig., December 1986 (Los Angeles), pages 266-269. In these techniques the HBTs are fabricated by mesa etching of epitaxially grown device structures. Deep mesas are required for collector contacts and possibly for device isolation, making it difficult to fabricate high density integrated circuits. More importantly, the devices also exhibit considerable parasitic capacitances, thus retarding their speed below their ultimate potential. Also, they are not readily compatible for integration with other GaAs devices such as metal-semiconductor-field-effect-transistors (MESFETs). While the emitter is self-aligned with the base, the collector is not truly self-aligned with either the emitter or base, and the base-collector overlap area is approximately three times the emitter-base junction area in the state-of-the-art HBTs. This increases the base-collector capacitance, thereby severely limiting the maximum operating frequency of the device. It is also difficult during fabrication to control the uniformity of the depth of the etch step to the base, which results in a poor yield. Another problem is that, since the emitter is only a little over a micron wide, the emitter contact tends to lap over and may short to the base contact. Finally, the device is non-planar because the collector contact is considerably below the level of the emitter contact.
The device discussed above is grown entirely by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), acting upon the surface. Another type of HBT is fabricated with a process that utilizes a two-step epitaxial deposition with an intervening selective ion implant of bases. It is described in Tully et al., "A Fully Planar Heterojunction Bipolar Transistor", IEEE Electron Device Letters, Vol. EDL-7, No. 11, November 1986, pages 615-17. While the surface of this device is substantially planar, the base and collector are not self-aligned. It is also relatively slow because of a high base-collector capacitance, and has a high resistance collector contact.
A fully planar HBT in an "emitter-down" configuration is disclosed in S. Evans et al., "GaAs HBT LSI/VLSI Fabrication Technology", 1987 GaAs IC Symposium Tech. Digest, 1987, pages 109-112. In this inverted device the base is doped by ion implantation through a top collector layer, with the emitter formed on the underside of the HBT. It is intended for high density digital logic applications where switching speeds are considerably slower than the state-of-the-art. It is not self-aligned, and is characterized by a high capacitance that makes it slow in speed.